MFJ-941E Versa Tuner II
1
MFJ VERSA TUNER II
GENERAL INFORMATION
The MFJ-941E is designed to match virtually any transmitter to any antenna, including dipoles, inverted-
vees, verticals, mobile whips, beams, random wires, and others fed by coax lines, balanced lines or a
single wire. An eight-position antenna-selector switch provides versatile antenna selection. The MFJ-
941E will handle up to 300 Watts of transmitter RF output power. The MFJ-941E employs a cross-
needle meter so forward power, reflected power and SWR may be read simultaneously.
CROSS-NEEDLE SWR/WATTMETER
The meter on the MFJ-941E may be used alone or with the tuner. Set the ANTENNA SELECTOR to
COAX 1 DIRECT or COAX 2 DIRECT to use the meter without the tuner. The MFJ-941E utilizes a
cross-needle meter so FORWARD power, REFLECTED power and SWR may be read simultaneously in
two ranges. FORWARD power may be read by setting the power range switch to HI (300 Watts) or LO
(30 Watts). Next read the power level on the FORWARD SCALE. REFLECTED power is shown
simultaneously on the REFLECTED SCALE. SWR is determined by observing the intersection point of
the two needles. No SWR sensitivity adjustment is needed to read SWR. The HI range is 300 Watts
FORWARD and 60 Watts REFLECTED. The LO range is 30 Watts FORWARD and 6 Watts
REFLECTED. The difference between the HI and LO scales readings is a factor of 10.
The meter lamp can be powered by a 12 Vdc source, such as the optional MFJ-1312B power supply. Use
a 2.1mm coaxial plug with the center conductor of the plug connected to the positive and the sleeve
connected to ground. The METER LAMP ON/OFF switch will activate the meter lamp.
ANTENNA SELECTOR
The ANTENNA SELECTOR switch utilizes eight positions. They are DUMMY LOAD, BALANCE
LINE, COAX 1, and COAX 2 positions for both BYPASS (direct) and TUNED (through) the tuner
operation. An external 50 Ohm dummy load can be connected to the EXT LOAD connector located at
the rear of the tuner. Do not continuously key into the dummy load for more than 2 minutes at a time.
CAUTION: Never use the MFJ-941E for OVER 300 Watts of RF output power, even in the
DIRECT or DUMMY LOAD positions.
INSTALLATION:
1.
The tuner should be placed in a location where operation will be convenient. The ceramic feed
through insulators will have high
RF voltages if random wire or balanced line operation is used.
These voltages can cause serious RF burns if touched when transmitting.
NOTE:
Locate the tuner so the rear is not accessible during operation.
2.
The MFJ-941E should be installed between the transmitter and antenna. A coaxial line should be
connected to the transmitter and the SO-239 coax TRANSMITTER connector on the back of the
tuner.
3. One or two coax-fed antennas may be connected to the SO-239 coax connectors marked COAX 1
or COAX 2. Coax 1 and Coax 2 antennas may be connected directly to the transmitter, bypassing
the tuner, by setting the ANTENNA SELECTOR switch to COAX 1 DIRECT or COAX 2
DIRECT, respectively.
MFJ-941E Versa Tuner II
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4. A random wire antenna may be connected to the five-way binding post marked WIRE. The
random length wire should be long, high, and as clear of surrounding objects as possible. For
optimum operation, the wire antenna should be a quarter wave-length or longer at the operating
frequency. Do NOT ground the random wire antenna. The tuner should be well-grounded to the
transmitter. A binding post marked GROUND is provided for ground connections.
5.
A balanced line-fed antenna may be connected to the two five-way binding posts marked
BALANCED LINE. A jumper wire from the WIRE binding post, as indicated by a dotted line
on the MFJ-941E, should be connected to one of the posts of the BALANCE LINE. This
couples the MFJ-941E to the balanced line through a 4:1 balun.
NOTE:
Either a balanced line or a random wire antenna may be connected to the MFJ-941E at one
time. If a random length wire is used, care should be taken to assure that no jumper wire is
between the WIRE binding post and the BALANCED LINE.
6. An external 50 Ohm dummy load may be connected to the EXT. DUMMY LOAD connector
located at the rear of the tuner.
MFJ-941E FRONT PANEL
0
1
2
3
4
5
6
7
8
9
10
0
1
2
3
4
5
6
7
8
9
10
TRANSMITTER
MATCHING
ANTENNA
MATCHING
INDUCTOR
SELECTOR
J
K
L
A
B
C
D
E
F
G
H
I
MFJ VERSA TUNER II
ANTENNA
SELECTOR
ON
POWER
OFF
300W
30W
MODEL MFJ-941E
LAMP
BAL.
LINE
WIRE
BAL.
LINE
WIRE
EXT.
DUMMY
LOAD
EXT.
DUMMY
LOAD
COAX 1COAX 1
COAX 2COAX 2
TUNED BYPASS
USING THE MFJ-941E
The INDUCTOR switch on the MFJ-941E represents maximum inductance at position "A" and the
minimum inductance at position "L". Lower inductance is needed at higher frequencies than at low
frequencies for the same impedance. The TRANSMITTER and ANTENNA controls represent
maximum capacitance at position 10. For optimum operation of the MFJ-941E, the transmitter must be
tuned to a 50 Ohm output impedance at the frequency of operation. The ANTENNA SELECTOR
switch should be set to DUMMY LOAD for tuning up the transmitter.
NOTE: The transmitter should always be tuned at a low output power.
After the transmitter is properly tuned, the ANTENNA SELECTOR should be set to the desired antenna
and the tuner adjusted for a minimum SWR as described below. DO NOT readjust the transmitter
loading control setting after loading it to 50 Ohms.
NOTE:
When using the MFJ-941E for receiving only, tune as described in Steps 1 and 2.
TUNER ADJUSTMENT
1. Set the TRANSMITTER and ANTENNA controls to 5. In this position the capacitors are half-
open.
2. Rotate the INDUCTOR control until maximum noise is obtained with the transceiver in the
receiving mode.
CAUTION: Do not operate the ANTENNA selector switch while transmitting!.
MFJ-941E Versa Tuner II
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3. While transmitting a steady state carrier (CW) alternately adjust TRANSMITTER and
ANTENNA controls for minimum SWR. Since both controls interact, the two controls can best be
adjusted by turning the TRANSMITTER control one small increment at a time, then rotating the
ANTENNA control for minimum SWR. Repeat this process until minimum SWR is obtained.
4. If a SWR of 1:1 is not achieved, the INDUCTOR control should be increased or decreased and
Step 3 repeated. If arcing should occur between capacitor plates, the INDUCTOR control should
be increased or decreased by one position, and Step 3 repeated. If SWR of 1:1 cannot be achieved,
Step 3 should be repeated for each INDUCTOR control position.
5.
After a minimum SWR is achieved, transmitter power may be increased to 300 Watts. The
VERSA TUNER II will reduce the SWR of most feed systems to 1:1. In some cases, a 1:1 SWR
is not achievable. In such cases, the length of the antenna may be increased or decreased to
improve SWR.
6. SWR of 1:1 may occur at more than one set of control settings on the MFJ-941E. When an SWR
of 1:1 is obtained, make sure that the transmitter power is relatively high. If transmitter power has
decreased substantially, try another INDUCTOR control setting and repeat Step 3.
TROUBLESHOOTING
If this tuner fails to tune, please double check all connections and follow the tuning procedures again.
Be sure you are using enough inductance (lowest letter usable for band) and have the capacitors open
far enough (highest front panel number).
If this tuner arcs at the rated power levels, please double check all connections and follow the tuning
procedures again. Be sure you are using the least amount of inductance and the greatest capacitance
possible to match the load on the operating frequency. If you are still unsuccessful, please read the
Antenna Matching Problems text below.
NOTE: If this tuner arcs when operating on the 160 meter band, it may be necessary to reduce
transmitter output power.
MATCHING ANTENNAS
Most matching problems occur when the antenna system presents an extremely high impedance to the
tuner. When the antenna impedance is much lower than the feedline impedance, an odd quarter-
wavelength feedline converts the low antenna impedance to a very high impedance at the tuner. A
similar problem occurs if the antenna has an extremely high impedance and the transmission line is a
multiple of a half-wavelength. The half-wavelength line repeats the very high antenna impedance at the
tuner. Incorrect feedline and antenna lengths can make an antenna system very difficult or
impossible to tune.
This problem often occurs on 80 meters if an odd quarter-wave (60 to 70 foot) open wire line is used to
feed a half-wave (100 to 140 foot) dipole. The odd quarter-wave line transforms the dipole's low
impedance to over three thousand Ohms at the tuner. This is because the mismatched feedline is an odd
multiple of 1/4 wavelength long. The line inverts (or teeter-totters) the antenna impedance.
A problem also occurs on 40 meters with this antenna example. The feedline is now a multiple of a half-
wave (60 to 70 foot) and connects to a full-wave high impedance antenna (100 to 140 foot). The half-
wave line repeats the high antenna impedance at the tuner. The antenna system looks like several
thousand Ohms at the tuner on 40 meters. The following suggestions will reduce the difficulty in
matching an antenna with a tuner:
MFJ-941E Versa Tuner II
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1.
Never center feed a half-wave multi-band antenna with a high impedance feedline that is close to
an odd multiple of a quarter-wave long.
2.
Never center feed a full-wave antenna with any feedline close to a multiple of a half-wave long.
3.
If a tuner will not tune a multi-band antenna, add or subtract 1/8 wave of feedline (for the band
that won't tune) and try again.
4.
Never try to load a G5RV or center fed dipole on a band below the half-wave design frequency. If
you want to operate an 80 meter antenna on 160 meters, feed either or both conductors as a
longwire against the station ground.
To avoid problems matching or feeding any dipole antenna with high impedance lines, keep the lines
around these lengths [ The worst possible line lengths are shown in brackets ]:
160 meter
dipole:
35-60, 170-195 or 210-235 feet.
[ Avoid 130, 260 ft ]
80 meter
dipole:
34-40, 90-102 or 160-172 feet.
[ Avoid 66, 135, 190 ft ]
40 meter
dipole:
42-52, 73-83, 112-123 or 145-155 feet.
[ Avoid 32, 64, 96, 128 ft ]
NOTE: Some trimming or adding of line may be necessary to accommodate higher bands.
WARNING: To avoid problems, a dipole antenna should be a full half-wave on the
lowest band. On 160 meters, an 80 or 40 meter antenna fed the normal way
will be extremely reactive with only a few Ohms of feedpoint resistance.
Trying to load an 80 meter (or higher frequency) antenna on 160 meters
can be a disaster for both your signal and the tuner. The best way to
operate 160 with an 80 or 40 meter antenna is to load either or both
feedline wires (in parallel) as a longwire. The antenna will act like a "T"
antenna worked against the station ground.
TECHNICAL ASSISTANCE
If you have any problem with this unit first check the appropriate section of this manual. If the manual
does not reference your problem or your problem is not solved by reading the manual, you may call MFJ
Technical Service at 662-323-0549 or the MFJ Factory at 662-323-5869. You will be best helped if you
have your unit, manual and all information on your station handy so you can answer any questions the
technicians may ask.
You can also send questions by mail to MFJ Enterprises, Inc., 300 Industrial Park Road, Starkville, MS
39759; by Facsimile to 662-323-6551; or by email to techinfo@mfjenterprises.com. Send a complete
description of your problem, an explanation of exactly how you are using your unit, and a complete
description of your station.
MFJ-941E Versa Tuner II
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